Compressed exfoliated graphite articles are known in the art, as are composite materials comprising resin-impregnated graphite sheets. These structures find utility, for example, in gasket manufacture.
In addition to their utility in gasket materials, graphite composites also find utility as heat transfer or cooling apparatus. The use of various solid structures as heat transporters is known in the art. For example, Banks, U.S. Pat. Nos. 5,316,080 and 5,224,030 discloses the utility of diamonds and gas-derived graphite fibers, joined with a suitable binder, as heat transfer devices. Such devices are employed to passively conduct heat from a source, such as a semiconductor, to a heat sink.
Graphite-based thermal management components offer several advantages in electronic applications and can help eliminate the potential negative impacts of heat generating components in computers, communications equipment, and other electronic devices. Graphite-based thermal management components include heat sinks, heat pipes and heat spreaders. All offer thermal conductivity comparable with or better than copper or aluminum, but are a fraction of the weight of those materials, and provide significantly greater design flexibility. Graphite-based thermal management products take advantage of the highly directional properties of graphite to move heat away from sensitive components. Compared to typical aluminum alloys used for heat management, the inventive graphite components can exhibit up to 300% higher thermal conductivity, with values comparable to copper (˜400 watts per meter degree Kelvin, i.e., W/mK) or greater. Further, aluminum and copper are isotropic, making it difficult to channel the heat in a preferred direction.
The graphite material for use in this invention is graphite material formed from compressed particles of exfoliated graphite.
The following is a brief description of graphite and the manner in which it is typically processed to form flexible materials. Graphite, on a microscopic scale, is made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially-flat, parallel, equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly-ordered graphite materials consist of crystallites of considerable size, the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites, by definition, possess anisotropic structures and thus exhibit or possess many characteristics that are highly directional, e.g., thermal and electrical conductivity and fluid diffusion.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the “c” direction. The graphites suitable for manufacturing flexible graphite articles possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be chemically treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been chemically or thermally expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension, can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, or the like (typically referred to as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible articles by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the graphite material, as noted above, has also been found to possess a high degree of anisotropy to thermal and electrical conductivity and fluid diffusion, somewhat less, but comparable to the natural graphite starting material due to orientation of the expanded graphite particles substantially parallel to the opposed faces of the material resulting from very high compression, e.g. roll processing. Material thus produced has excellent flexibility, good strength and a very high degree or orientation. There is a need for processing that more fully takes advantage of these properties.
Briefly, the process of producing binderless anisotropic graphite material, e.g. sheets, articles, web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a “c” direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, integrated graphite article. Typically, the article formed is a flexible, relatively thin (i.e., 5 mm or less) sheet, although thicker articles are also capable of being produced in this manner. The expanded graphite particles that generally are worm-like or vermiform in appearance will, once compressed, maintain the compression set and alignment with the opposed major surfaces of the sheet. Properties of the article may be altered by coatings and/or the addition of binders or additives prior to the compression step. See U.S. Pat. No. 3,404,061 to Shane, et al. The density and thickness of the material can be varied by controlling the degree of compression.
Lower densities are advantageous where surface detail requires embossing or molding, and lower densities aid in achieving good detail. However, higher in-plane strength, thermal conductivity and electrical conductivity are generally favored by more dense sheets. Typically, the density of the material will be within the range of from about 0.04 g/cm3 to about 1.4 g/cm3.
Graphite material made as described above typically exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the material, with the degree of anisotropy increasing upon roll pressing to increased density. In roll-pressed anisotropic material, the thickness, i.e. the direction perpendicular to the opposed, parallel surfaces comprises the “c” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal, electrical and fluid diffusion properties of the material are very different, by orders of magnitude typically, for the “c” and “a” directions.